The use of various materials for the sorption of gases is well known. Charcoal and zeolites are examples of non-metallic gas sorbers. Metallic gas sorbers or getters are also commonly used. Barium is particularly well known for its ability to sorb large quantities of gas very rapidly. Due to the high reactivity of barium metal, it is usually handled in the form of an alloy, with aluminum, for instance, in about a 50% weight ration. When it is desired to start sorption of gases in, for example, a thermionic valve or a television picture tube, the barium is released by heating the barium-aluminum alloy, whereupon barium evaporates, depositing or condensing upon the walls of the device in which it is being used. The evaporated film of barium is then capable of sorbing gases, maintaining a high vacuum within the device. In certain circumstances, it is undesirable to have an evaporated metallic film. In this case, use is made of a metal or alloy which is capable of sorbing gases even though the metal has not been evaporated. Such getter materials are called non-evaporable getters. One example is described in U.S. Pat. No. 2,926,981 and relates to the use of zirconium-titanium alloys. A particularly well known non-evaporable getter alloy of zirconium with aluminum is described in U.S. Pat. No. 3,203,901. Usually these gettering alloys are covered with a passivating layer of oxides and nitrides, which are removed by means of a heat treatment or activating process before the alloy is capable of sorbing gas. The activation process usually involves heating the getter metal to temperatures of 800.degree.-900.degree. C. for a period of tens of seconds to a few minutes. If the getter metal is not activated, it may still be capable of selectively sorbing gas if the metal is simply heated to a given temperature. This may be a desirable property. For instance, A Barosi and E. Rabusin in Japan J. Appl. Phys. Suppl. 2, Pt. 1, 1974 p. 49-52, describes the use of the aforementioned zirconium-aluminum alloy in high intensity discharge lamps. These lamps have a nitrogen gas filling within the outer glass envelope. Hydrogen impurity is dangerous to lamp operation. It is found that the use of a non-activated zirconium-aluminum alloy, if heated to about 400.degree. C..+-.50.degree. C. without a prior activation step, is capable of removing the undesirable hydrogen without continuously sorbing nitrogen. Unfortunately, in some lamps, it may be difficult to find a position to mount the getter alloy where it can reach 400.degree. C. during lamp operation. If such a position is available, it may only be possible to maintain the temperature if the lamp is in a predetermined position. This limits the flexibility of use of the lamp.
Intermetallic compounds such as Zr.sub.2 Ni have also been used as selective getters as described in U.S. Pat. No. 4,071,335. However, the particular advantage of Zr.sub.2 Ni is its ability to sorb water vapour without releasing hydrogen. The speed with which it sorbs hydrogen at low temperatures is very low. In an atmosphere of hydrogen at a pressure of 1 torr and at a temperature of 250.degree. C., non-activated Zr.sub.2 Ni sorbs only 2.2 cc torr in a period of 3 hours.
G. Kuus in Digest No. 1978/29 of the IEE Electronics Division describes the use of a not better specified "Zr-Ni getter" as a hydrogen getter inside the outer bulb of a high pressure metal iodide lamp.